Electrical generators are often driven from power sources such as steam turbines. Steam turbines generally include a shaft line comprising several turbine stages and a generator, forming a complex system.
In normal operation, the shaft line rotates at a given synchronous frequency, for example 25 Hz, 30 Hz, 50 Hz or 60 Hz.
However, when rotating, the shaft line may also be subjected to torsional oscillations. These torsional oscillations may result from disturbances internal to the generator or from disturbances external to the generator along the electrical system supplied by the generator.
Such torsional oscillations may in particular appear when the generator is used for the delivery of energy to a grid network through long capacitive power transmission lines poorly interconnected. Indeed, when long transmission lines are required for the delivery of energy to a grid network, series capacitive compensation is generally used to reduce the angular deviation and the voltage drop, and thus increase the stability of the line. However, the compensating capacitors result in the presence of certain resonant frequencies in the grid network, which may cause torsional oscillations of the shaft line.
Torsional oscillations induce a variation in the speed of the shaft line proportional to the torsion angle of the shaft line. Torsional oscillations thus result in a non-uniform rotation frequency along the shaft line.
In certain conditions, when the electrical resonance (as grid electrical resonance) cooperates with a resonance frequency of the shaft line, resonant torsional oscillations may occur. The resonant torsional oscillations occur at resonance frequencies which are lower than the synchronous frequency of the shaft line, and are thus called sub synchronous resonances.
Torsional oscillations cause torsional stresses at critical locations along the shaft line which may damage the shaft line, and in some instances may even result in the breakage of the shaft line.
In order to solve this problem, it has been proposed to monitor the strain on the shaft line by means of strain gauges placed on the shaft. However, this solution has low reliability over time and may only be used for punctual assessments, and not for permanent monitoring of the shaft.
It has thus been proposed in US 2005/0183504 A1 to monitor the strain on the shaft line in an indirect manner, for example by analyzing the power supplied to the network grid in order to evaluate the torsional oscillations of the shaft line. However, this method implies the use of extra devices to measure the power supplied.
It is therefore an object of the invention to provide a system and a method for monitoring the torsional oscillations on a shaft line which has a sufficient accuracy and can be simply implemented on an existing shaft line, in a cost effective manner.